Spectral and isotopic characteristics of particulate organic matter in a subtropical estuary under the influences of human disturbance

Particulate organic matter (POM) in the coastal zone plays an important role in global carbon cycle, yet its spectral properties under human disturbance are not well understood. This study investigated the source and dynamics of POM in a typical coastal area (Minjiang Estuary, China) under the influences of estuarine mixing, urbanization and dam construction, based on spectral and isotopic analysis. The yield of particulate organic carbon from the watershed (0.38 g m−2 yr−1) was in part limited by the historical decrease in sediment discharge downstream hydropower station. The absorption and fluorescence indices of POM suggested an important contribution from biological production with a low humic content. The importance of river and estuarine production was supported by elemental and isotopic analysis, in particular for N-bound constituents of POM. Three humic-like and two protein-like fluorescent components were identified from POM using fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). They showed weak correlations with salinity, suggesting non-conservative behaviors of fluorescent POM during the estuarine mixing. Principal component analysis (PCA) identified two principal factors, which were related to the level of chromophoric POM (PC1) and biological production (PC2), respectively. The PCA results indicated stronger influence of biological production in the estuarine zone and potential effect of sediment re-suspension and/or wastewater discharge in the urban zone. Overall, our results demonstrated significant influences of human disturbance on the flux, source and dynamics of POM in the coastal zone, which could be assessed by spectral and stable isotopic analysis.